How a Simple Water Filter Works

You have created a simple water filter! The sand and gravel particles act as a multi-layered sieve, trapping larger material and debris from travelling down with the water as it moves through the bottle.

Filtering like this occurs in the natural environment every day. As rainwater soaks into the ground, it passes through different layers of soil and rock. This process relies on porosity (the tiny spaces between the grains) and permeability (how easily water can flow through those spaces). While the sand traps solid particles, the chemical properties of some soils can even help neutralise certain impurities.

There are many communities that rely on underground aquifers which have accumulated over millions of years as water has filtered through the soil to the bedrock. A classic example is Australia’s Great Artesian Basin, which covers much of Queensland and South Australia, with extensions into the Northern Territory and NSW. It is one of the largest and deepest artesian basins in the world!

You can create a simple model of soil horizons that produce these natural water filters by alternating different types of sand and gravel in a clear container to see how the “clarity” of the water improves as it passes through each layer.

Variables to test

Find out more on variables here.

• Mesh Density
  Try different size sieve meshes by changing the dishcloth type at the bottom of the filter. Does a tighter weave result in clearer water?
• Particle Size
  What happens if you change the size of the sand and gravel grains? Does using very fine sand slow down the filtration process too much, or does it make the water cleaner?
• Layer Order
  Does it matter if the gravel is on top of the sand or vice versa? Consider which arrangement would trap the largest “bits” first to prevent the filter from clogging.